1. Field of the Invention
The present invention relates to a semiconductor device, in particular, a semiconductor device having a structure of a hetero-bipolar transistor, and a method of forming the semiconductor device.
2. Related Prior Arts
Continuous requests for a semiconductor device to be operable in a higher speed have become stronger as communication devices have become faster. One solution for such requests is provided by a hetero-junction bipolar transistor (HBT) primarily made of compound semiconductor materials. Various prior arts have reported an HBT including, on a semi-insulating GaAs substrate, an n-type GaAs collector layer, a p-type GaAs base layer, and a composite emitter layer of an n-type InGaP and an n-type AlGaAs, where the emitter layer has energy band gap wider than those of the base layer and the collector layer. When an HBT becomes operable in frequencies exceeding several scores of giga-hertz, sometimes reaching a hundred of giga-hertz, parasitic components around electrodes of the HBT are necessary to be further reduced.